CBSE GRADE 12 Energy Band : The band that has large number of closely spaced

PHYSICS STUDY MATERIAL energy levels in a very small energy range is known as energy band.
CHAPTER 14 SEMI CONDUCTOR ELECTRONICS
Valence Band :
Syllabus ( 2024 – 2025 ) : Energy bands in conductors, semiconductors
 The energy band formed due to the valence orbitals is called
and insulators (qualitative ideas only) Intrinsic and extrinsic
valence band (VB).
semiconductors- p and n type, p-n junction Semiconductor diode - I-V
 The valence band is the lower band that is completely filled with
characteristics in forward and reverse bias, application of junction diode
electrons.
-diode as a rectifier
Conduction Band :

Energy band diagram of solids : Let us consider an atom with one  The energy band formed due to the unoccupied orbitals is called

electron in the outermost orbit. It means that the number of valence conduction band (CB).

electrons is one. When two such atoms are brought close to each other,  The conduction band is the upper band that is unfilled

the valence orbitals are split up into two. Similarly the unoccupied  The electrons can jump from VB to CB when energised.
orbitals of each atom will also split up into two. The electrons have the
Forbidden Energy Gap : The energy gap between the valence band
choice of choosing any one of the orbitals as the energy of both the
and the conduction band is called forbidden energy gap (Eg). Electrons
orbitals is the same. When the third atom of the same element is brought
cannot exist in the forbidden energy gap.
to this system, the valence orbitals of all the three atoms are split into
 EV represents the maximum energy of the valence band and
three. The unoccupied orbitals also will split into three.
 EC represents minimum energy of the conduction band.
In reality, a solid is made up of millions of atoms. When millions of atoms  Hence the forbidden energy gap, Eg = EC– Ev
are brought close to each other, the valence orbitals and the unoccupied
orbitals are split according to the number of atoms. In this case, the
energy levels will be closely spaced and will be difficult to differentiate
the orbitals of one atom from the other and they look like a band as
shown in Figure.

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 CLASSIFICATION OF MATERIALS BASED ON ENERGY BAND TYPES OF SEMI CONDUCOTRS

Conductors : In conductors, the valence band and conduction band

overlap. Hence, electrons can move freely into the conduction band from
valence band. So, conduction is possible even at low temperatures.

Semiconductors : In semiconductors, forbidden energy gap is small

( Eg < 3 eV ) At a finite temperature. A small increase in temperature is
enough to transfer electrons from valence band to conduction band. The
conductivity of the semiconductors is less as that of the conductors.

Eg for germanium = 0.72 Ev Eg for silicon = 1.1 eV

Intrinsic semiconductors : A pure semiconductor without any impurity
At zero Kelvin, semiconductor behaves as insulator is called as intrinsic semiconductor. Here, impurity means presence of
Insulator : In Insulator the valence band and the conduction band are any other foreign atom in the crystal lattice.
separated by a large energy gap. The forbidden energy gap is
Each Germanium atom has four electrons in the outermost orbit and is
approximately 6 eV in insulators. The gap is very large that electrons
covalently bonded with four neighbouring atoms to form the lattice. A
from valence band cannot move into conduction band even on the
small increase in temperature is sufficient enough to break some of the
application of strong external electric field or the increase in temperature.
covalent bonds and release the electrons free from the lattice .As a
Therefore, the electrical conduction is not possible..
result, electrons move from VB to CB. The vacancies produced in the
valence band are called holes. As the holes are deficiency of electrons,
they are treated to possess positive charges. Hence, electrons and holes
are the two charge carriers in semiconductors.
In intrinsic semiconductors, the number of electrons in the conduction
band is equal to the number of holes in the valence band.

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The total current I is always the sum of the electron current and the hole phosphorus, arsenic and antimony. During the process of doping, a few
current. I = Ie + Ih. of the silicon atoms are replaced by pentavalent dopants .The dopant
has five valence electrons while the silicon atom needs only four for
bonding .

The fifth valence electron of the impurity atom is loosely attached with
the nucleus and they form a energy band just below thw conduction
band and is called the donor energy level as a result the forbidden
energy gap decreases.

An intrinsic semiconductor behaves like an insulator at 0 K At room temperature, these electrons can easily move to the conduction
band with the absorption of thermal energy. Therefore, each impurity
DOPING: The process of adding impurities to the intrinsic semiconductor atom provides one extra electron to the conduction band in addition to
is called doping. the thermally generated electrons. These thermally generated electrons
It increases the concentration of charge carriers (electrons and holes) in leave holes in valence band.
the semiconductor and in turn, its electrical conductivity increases. The Hence, the majority carriers of current in an n-type semiconductor
impurity atoms are called dopants and its order is approximately 100 are electrons and the minority carriers are holes.
ppm (parts per million).

Extrinsic semiconductors : On the basis of the type of impurity added,

extrinsic semiconductors are classified into
i) n-type semiconductor ii) p-type semiconductor

n-type semiconductor :
A n-type semiconductor is obtained by doping a pure silicon (or
ne >> nh
germanium) crystal with pentavalent impurity atoms such as

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p -type semiconductor : COMPARISION OF ENERGY BAND DIAGRAM
( n type & p type semi conductor )
In p-type semiconductor, trivalent impurity atoms such as boron,
aluminium, gallium and indium are added to the silicon (or germanium)
crystal. The dopant has only three valence electrons but silicon needs
four to form a covalent bond. So there is electron vacancy.. This electron
vacancy present is called as a hole.

To make complete covalent bonding with all four neighbouring atoms,

the dopant is in need of one more electron. These dopants can accept
electrons from the neighbouring atoms. Therefore, this impurity is called
an acceptor impurity. The energy level of the hole created by each
impurity atom is just above the valence band and is called the acceptor
energy level, as a result the forbidden energy gap decreases.

DIFFERENTIATE INTRINSIC & EXTRINSIC SEMI CONDUCTOR

For each acceptor atom, there will be a hole in the valence band; this is
in addition to the holes left by the thermally generated electrons. In such
an extrinsic semiconductor, holes are the majority carriers and
electrons are minority carriers

nh >> ne
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 DIFFERENTIATE n TYPE & p TYPE SEMI CONDUCTOR Depletion region :
Due to the diffusion of electrons and holes, a layer of positive charge
donor atoms in n-region and a layer of negative charge acceptor atoms
in p-region is created. This positive and negative charge regions on both
sides of PN junction will form a region with immobile ions called as
depletion region.

Drift current :

An electric field is set up between the positively charged layer in the

n-side and the negatively charged layer in the p-side in the depletion
region. This electric field makes electrons in the p-side drift into the n
side and the holes in the n-side into the p-side.
The electric current produced due to the motion of the minority charge
PN JUNCTION
carriers by the electric field is known as drift current.
A single piece of semiconductor crystal is suitably doped such that its
The diffusion current and drift current flow in opposite directions.
one side is p-type semiconductor and the other side is n-type
semiconductor. The contact surface between the two sides is called p-n
junction.

Diffusion current :
Whenever p-n junction is formed, some of the free electrons diffuse from
the n-side to the p-side while the holes from the p-side to the n-side. The
diffusion happens since n-side has higher electron concentration and the
p-side has higher hole concentration.
The diffusion of the majority charge carriers across the junction
gives rise to an electric current, called diffusion current.

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Though drift current is less than diffusion current initially, equilibrium is BIASING A DIODE
reached between them at a particular time.
Hence at equilibrium, there is no net electric current across the Biasing means providing external energy to charge carriers to overcome
junction. Thus, a p-n junction is formed. the barrier potential and make them move in a particular direction.
The external voltage applied to the p-n junction is called bias voltage.
Junction Potential or Barrier Potential : Depending on the polarity of the external source to the p-n junction, we

The immobile ions on both sides of the junction establish an electric have two types of biasing:

potential difference across the junction.

i) Forward bias ii) Reverse bias

FORWARD BIAS :
If the positive terminal of the external voltage source is connected to the
p-side and the negative terminal to the n-side, it is called forward biased

This difference in potential across the depletion layer is called the

barrier potential (Vb)

PN JUNCTION DIODE
A p-n junction diode is formed when a p-type semiconductor is fused
with an n-type semiconductor.

The application of a forward bias potential pushes electrons in the n-side

and the holes in the p-side towards the junction. This initiates the
recombination with the ions near the junction which in turn reduces the
width of the depletion region

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In addition, the accelerated electrons experience a strong attraction by
The net effect is the widening of the depletion region leading to an
the positive potential applied to the p-side and similarly, holes in the
increase in the barrier potential. Yet, a small current flows across the
p-side towards the n-side. When the applied voltage is increased, the
junction due to the minority charge carriers called as reverse saturation
width of the depletion region and hence the barrier potential are further
current. The reverse saturation current is independent of the applied
reduced.
voltage.
V-I CHARACTERSITICS OF A PN JUNCTION DIODE
This results in a large number of electrons passing through the junction
resulting in an exponential rise in current through the junction. FORWARD CHARACTERISTICS :

It is the study of the variation in current through the diode with respect to
REVERSE BIAS :
the applied voltage across the diode when it is forward biased.
If the positive terminal of the battery is connected to the n-side and the
negative terminal to the p-side, the junction is said to be reverse biased
An external resistance (R) is used to limit the flow of current through the
diode. The forward bias voltage and the corresponding forward bias
current are noted and a graph is plotted. This graph is called the forward
V-I characteristics

As the positive terminal is connected to the n-type material, the electrons

in the n-side are attracted towards the positive terminal and the holes in Initially when the voltage is raised there is no rise in the current
the p-side are attracted by the negative terminal. This increases the
immobile ion concentration at the junction.
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Forward voltage (or) Threshold voltage (or) cut-in voltage (or) This reverse current is independent of the voltage up to a certain
knee voltage : voltage, known as breakdown voltage.

The current flow is negligible when the applied voltage is less than the
threshold voltage. Beyond the threshold voltage, increase in current is
significant even for a small increase in voltage. it is approximately 0.3 V
for germanium and 0.7 V for silicon.

The graph clearly infers that the current flow is not linear and is
exponential. Hence it does not obey Ohm’s law. ( Non Ohmic device )
Thus the diode behaves as a conductor when it is forward biased.
DIODE AS A RECTIFER
Breakdown voltage :
However, if the applied voltage is increased beyond a rated value, it will Rectification : The process in which alternating voltage or

produce an extremely large current which may destroy the junction due alternating current is converted into direct voltage or direct current

to overheating. This is called as the breakdown of the diode and the is known as rectification.

voltage at which the diode breaks down is called the breakdown voltage. The device used for this process is called as rectifier. There are two
types of rectifier

REVERSE CHARACTERISTICS : HALF WAVE RECTIFIER

In the reverse bias, the p-side of the diode is connected to the negative The half wave rectifier circuit consists of a transformer, a p-n junction

terminal and n-side to the positive terminal of the dc power supply. diode and a resistor
In a half wave rectifier circuit, either a positive half or the negative half of

A graph drawn between the reverse bias voltage and the current across the AC input is passed through by the diode while the other half is

the junction is called the reverse characteristics of a p-n junction diode. blocked. Only one half of the input wave is rectified. Therefore, it is

Under this bias, a very small current in μA flows across the junction. called half wave rectifier. Here, a p-n junction diode acts as a rectifier

This is due to the flow of the minority charge carriers and is called the diode.

leakage current or reverse saturation current.

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During the positive half cycle : When the positive half cycle of the AC During the negative half cycle : When the negative half cycle of the
input signal passes through the circuit, terminal A becomes positive with AC input signal passes through the circuit, terminal A is negative with
respect to terminal B. The diode is forward biased and hence it respect to terminal B. Now the diode is reverse biased and does not
conducts. conduct. Hence no current passes through RL The reverse saturation
current in a diode is negligible.
The current flows through the load resistor RL and the AC voltage
developed across RL constitutes the output voltage V The output of the half wave rectifier is not a steady DC voltage but a
pulsating wave. This pulsating voltage cannot be used for electronic
equipments.
A constant or a steady voltage is required which can be obtained with
the help of filter circuits and voltage regulator circuits.

Efficiency (η) is the ratio of the output DC power to the AC input

power supplied to the circuit. Its value is 40.6 %

FULL WAVE RECTIFIER

The positive and negative half cycles of the AC input signal are rectified
in this circuit and hence it is called the full wave rectifier.

It consists of two p-n junction diodes, a centre tap transformer and a load
resistor R. The centre is usually taken as the ground or zero voltage
reference point. With the help of the centre tap transformer, each diode
rectifies one half of the total secondary voltage.

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During positive half cycle : During negative half cycle : When the negative half cycle of the AC
When the positive half cycle of the AC input signal passes through the input signal passes through the circuit, terminal B becomes positive, and
circuit, terminal A is positive and B is at negative potential. This forward A is at negative potential. This forward biases diode D2 and reverse
biases diode D1 and reverse biases diode D2. Hence, being forward biases diode D1. Hence, being forward biased, diode D2 conducts and
biased, diode D1 conducts and current flows along the path. current flows along the path.
Though both half cycles of AC input are rectified, the output is still
pulsating in nature. The efficiency (η) of full wave rectifier is twice
that of a half wave rectifier and is found to be 81.2 %. The output of
the full wave rectifier is a pulsating wave. A constant or a steady voltage
is required which can be obtained with the help of filter circuits and
voltage regulator circuits.

FILTER CIRCUIT : A Single Capacitor of high capacitance is

connected across the rectifier to work as filter

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Class work Numericals :
28
1. Suppose a pure Si crystal has 5×10 atoms m−3. it is doped
by 1 ppm concentration of pentavalent As. Calculate the
1
number of electrons and holes. Given that ni =1.5×10
[ Ans : 4.5×10 9 / m3 , N type semiconductor ]

5. In the given diagram, which bulb out of B1 and B2 will glow

2. In half-wave rectification, what is the output frequency if the and why ?
input frequency is 50 Hz. What is the output frequency of a
full-wave rectifier for the same input frequency
[ Ans : foutout = 100 Hz ]

3. Assuming that the two diodes D1 and D2 used in the electric

circuit shown in the figure are ideal, find out the value of the DO YOU KNOW …. ?
current flowing through 2.5 Ω resistor Why a Centre tap transformer is used ?

The central tapping of the transformer is used to make both the

diodes conduct during alternate half cycles. In a center-tapped
full-wave rectifier, the center-tap is used to divide the
transformer secondary voltage into two halves.

4. Assuming that the two diodes D1 and D2 used in the electric

circuit shown in the figure are ideal, find out the value of the
current flowing through 2 Ω resistor

For your final exams

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